When mIRC window was maximized before exiting, window size is restored on next startup.
Unfortunatelly that doesn't happen if mIRC was iconified in tray before closing it. On next startup, mirc not only opens minimized in tray but also when clicked main window is displayed using its default size and position (not maximized).

Checked tray options:
"Place mIRC in tray when minimized"
"Animate icon..."
"Single click..."

Thanks this has been fixed for the next version.